1. Field of the Invention
The present invention relates generally to electrophotographic apparatuses, process cartridges, and electrophotographic photosensitive members. Particularly, the present invention relates to an electrophotographic apparatus employing an injection charging method and a toner recycling system and to a process cartridge and an electrophotographic photosensitive member therefor.
2. Description of the Related Art
A typical electrophotographic apparatus comprises an electrophotographic photosensitive member, a charging unit for charging the surface of the electrophotographic photosensitive member, an exposing unit for exposing the charged surface of the electrophotographic photosensitive member so as to form an electrostatic latent image, a developing unit for developing the electrostatic latent image using a toner so as to form a toner image, a transferring unit for transferring the toner image onto a recording medium such as paper, and a fixing unit for fixing the toner image on the recording medium.
A corona discharger has been commonly used as the charging unit. A contact discharging unit that charges the electrophotographic photosensitive member by putting a charging member having a voltage applied thereto into contact with the electrophotographic photosensitive member is also available. Compared with the corona discharger, the contact discharging unit has advantages of low ozone emission and low power consumption.
In a typical contact charging unit, a conductive charging member is placed in contact with an electrophotographic photosensitive member, and a predetermined charging bias is applied to the charging member, hereinafter referred to as the contact charging member, so as to charge the surface of the electrophotographic photosensitive member with a predetermined polarity and potential. Examples of contact charging members include rollers, fur brushes, magnetic blushes, and blades.
The contact charging unit includes two types of charging mechanisms, namely, a discharge charging mechanism and a direct-injection charging mechanism.
The discharge charging mechanism charges the surface of the electrophotographic photosensitive member by a discharge occurring in a minute gap between the contact charging member and the electrophotographic photosensitive member.
In discharge charging, application of a voltage higher than a charge potential to the contact charging member is necessary since the contact charging member and the electrophotographic photosensitive member have a particular discharge threshold value. Moreover, discharge charging inevitably generates discharge products, although the amount of the discharge products is significantly less than that of corona dischargers, and thus adverse effects caused by activated ions, such as ozone, cannot be completely avoided.
The direct-injection charging mechanism has been suggested in Japanese Patent Laid-Open No. 6-3921, for example. In the direct-injection charging mechanism, electrical charges are directly injected into the electrophotographic photosensitive member from the contact charging member to charge the surface of the electrophotographic photosensitive member. More particularly, a voltage is applied to the contact charging member so as to inject electrical charge into a charge sustaining section, such as the trap level, on the surface of the electrophotographic photosensitive member or conductive particles in charge injection layers and to charge the surface of the electrophotographic photosensitive member.
In a charging unit comprising a direct-injection charging mechanism, hereinafter referred to as a xe2x80x9cdirect-injection charging unitxe2x80x9d, a contact charging member is put into contact with the surface of the electrophotographic photosensitive member so as to directly inject electrical charges into the surface of the electrophotographic photosensitive member without utilizing a discharge phenomenon, i.e., essentially without requiring a discharge mechanism.
Accordingly, the direct-injection charging unit can charge the electrophotographic photosensitive member at an electrical potential corresponding to the voltage applied to the contact charging member even if the voltage applied to the contact charging member is below the discharge threshold value. In other words, since the discharge phenomenon is not dominant, the voltage for charging applied to the contact charging member need only have the charge potential desired at the surface of the electrophotographic photosensitive member.
Moreover, the direct-injection charging unit does not generate activated ions and is thus free from the problems caused by the discharge products, such as ozone.
A charging brush or a charging magnetic brush is expensive for use as the contact charging member. In a contact charging member comprising a magnetic brush, charged carriers often leak from a charging sleeve during an electrophotographic process. A roller, hereinafter referred to as a xe2x80x9ccharging rollerxe2x80x9d, is preferably used as the contact charging member.
However, direct-injection charging is difficult with a simple structure using a charging roller or a brush, and may result in the fogging of images, i.e., development of a blank background in reversal development, and nonuniform charging.
In view of the above problems, the following direct-injection charging unit has been suggested.
The suggested direct-injection charging unit comprises a contact charging member including charged particles, which come into contact with an electrophotographic photosensitive member, and a charged particle support for supporting the charged particles. The charged particle support has a conductive and elastic surface. With this direct-injection charging unit, sufficient contact and uniform charging are possible during direct-injection charging.
An electrophotographic apparatus comprising the above direct-injection charging unit may be of a toner recycle type. In such an electrophotographic apparatus, the charging unit also functions as means for supporting residual toner on the surface of the electrophotographic photosensitive member after transfer by a transfer unit so as to normalize the charge of the residual toner (primary recovering), and for returning the normalized toner onto the surface of the electrophotographic photosensitive member. The developing unit also functions as means for recovering the normalized residual toner returned onto the surface of the electrophotographic photoresistor by the charging unit.
Japanese Patent Publication No. 7-99442 discloses a contact charging apparatus in which a powder is applied on the face of a charging member that comes into contact with the surface of an electrophotographic photosensitive member so as to uniformly charge the contact charging apparatus. Although ozone products are reduced compared to a corona charger such as scorotron, charging is based on corona charging caused by discharging. Moreover, a direct current voltage superimposed by an alternating current voltage is applied to more reliably achieve uniformity in charging. Thus, the amount of the discharge products is still large.
However, an electrophotographic apparatus comprising a direct-injection charging unit as means for charging an electrophotographic photosensitive member and employing a toner recycling system suffers from the following problem. In the recycle system, toner recovered by the charging unit during primary recovering has an electrical potential of the same polarity as that of the voltage applied to the charging unit. The toner is returned to the surface of the electrophotographic photosensitive member by the potential difference relative to the surface of the electrophotographic photosensitive member. In such a system, an excessive amount of toner may be stored-in the charging unit after several uses depending on the performance of recycling, thereby damaging the electrophotographic photosensitive member. Even if the performance of toner recycle is satisfactory, imaging failure such as ghost may occur from an early stage.
The inventors have found through extensive research that both improved efficiency in toner recycling and prevention of ghosting can be achieved by adjusting the difference in charged potential between a first rotation and a second rotation of the electrophotographic photosensitive member of the electrophotographic apparatus in a predetermined range.
The recovery of the residual toner is done by a charging unit (primary recovery) and by a developing unit because of the difference between the surface potential of the electrophotographic photosensitive member and the voltage applied to the charging unit and the developing unit.
Accordingly, if the difference in the charged potential between the first rotation and the second rotation of the electrophotographic photosensitive member is excessively small, the difference in the potential between the surface of the electrophotographic photosensitive member and the charging unit also becomes small. As a result, the residual toner cannot be properly ejected onto the surface of the electrophotographic photosensitive member from the charging unit. The residual toner accumulates at the contact charging member of the charging unit, causing contamination of the charging unit. Contamination of the charging unit causes imaging failure and damages the surface of the electrophotographic photosensitive member.
When the difference in charged potential is excessively large, if the unit is used in a reversal development process in which a dark potential section is undeveloped and a light potential section is developed, the section illuminated during a previous printing operation remains. When a full-page black image or a halftone image is subsequently processed, the section illuminated during the last printing appears dark or comes off blank. In other words, a ghost may be generated.
The present invention is made based on the knowledge described above.
An aspect of the present invention provides an electrophotographic apparatus including a toner recycling system, the apparatus including:
an electrophotographic photosensitive member including a support and a photosensitive layer on the support;
a charging means for charging the surface of the electrophotographic photosensitive member;
an exposing unit for exposing the charged surface of the electrophotographic photosensitive member to form an electrostatic latent image;
a developing unit for developing the electrostatic latent image with toner so as to form a toner image;
a transferring unit for transferring the toner image onto a recording medium; and
a fixing unit for fixing the toner image on the recording medium,
the charging unit including a contact charging member that comes into contact with the surface of the electrophotographic photosensitive member, the surface of the electrophotographic photosensitive member being charged by the direct injection of electrical charge from the contact charging member to the electrophotographic photosensitive member, the contact charging member including charged particles that come into contact with the electrophotographic photosensitive member and a charged particle support for supporting the charged particles, the surface of the charged particle support being conductive and elastic,
the charging unit also functioning as a unit for supporting the residual toner on the surface of the electrophotographic photosensitive member after transfer, for normalizing the electrical charge of the residual toner, and for returning the residual toner onto the surface of the electrophotographic photosensitive member,
the developing unit also functioning as a unit for recovering the normalized toner returned onto the surface of the electrophotographic photosensitive member,
wherein the absolute value of the charge potential difference between a first rotation and a second rotation of the electrophotographic photosensitive member is higher than 5 V and lower than 70 V.
Another aspect of the present invention provides a process cartridge which is detachably attachable to a main body of an electrophotographic apparatus, the process cartridge including:
an electrophotographic photosensitive member including a support and a photosensitive layer;
a charging unit for charging the surface of the electrophotographic photosensitive member;
a developing unit for developing an electrostatic latent image with toner so as to form a toner image, the electrostatic latent image being formed on the charged surface of the electrophotographic photosensitive member by using an exposing unit of the electrophotographic apparatus;
a transferring unit for transferring the toner image onto a recording medium; and
a fixing unit for fixing the toner image on the recording medium,
the charging unit including a contact charging member that comes into contact with the surface of the electrophotographic photosensitive member, the surface of the electrophotographic photosensitive member being charged by direct injection of electrical charge from the contact charging member to the electrophotographic photosensitive member, the contact charging member including charged particles that come into contact with the electrophotographic photosensitive member and a charged particle support for supporting the charged particles, the surface of the charged particle support being conductive and elastic,
the charging unit also functioning as a unit for supporting the residual toner on the surface of the electrophotographic photosensitive member after the toner image formed on the surface of the electrophotographic photosensitive member by using the developing unit is transferred onto the recording medium through the transferring unit, for normalizing the electrical charge of the residual toner, and for returning the residual toner onto the surface of the electrophotographic photosensitive member,
the developing unit also functioning as a unit for recovering the normalized toner returned onto the surface of the electrophotographic photosensitive member,
wherein the absolute value of the charge potential difference between a first rotation and a second rotation of the electrophotographic photosensitive member, is higher than 5 V and lower than 70 V.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.